Preheat oven to 350 degrees F. Line a 13x9 inch baking pan with foil leaving a slight overhang; spray foil with cooking spray.
Whisk flour and next 4 ingredients in medium bowl and set aside.
In a large mixing bowl, using an electric mixer, beat brown sugar and butter until fluffy, about 2 minutes. Add eggs 1 at a time, beating well after each addition. Beat in vanilla. Stir in flour mixture, alternately with buttermilk. Stir in water (or coffee). Let batter cool if it is warm from the coffee, then stir in the chocolate chips.
Pour batter into pan and bake until tester inserted into center of cakes comes out clean, about 225-30 minutes or until cake springs back when touched. Set pan on a rack and cool completely. Wrap tightly in foil and freeze until ready to use.
Make Frosting. Using the top of a double boiler or the microwave, melt the chocolate and butter together. Stir in the espresso powder, then transfer melted chocolate mixture to the bowl of a stand mixer.
In a food processor, process sugar, cocoa, remaining butter and salt until thoroughly mixed. Scrape sugar mixture into the stand mixer bowl with the melted chocolate mixture and beat until smooth. Stir in cream.
Place the mixing bowl in refrigerator and chill until very cold. When chilled, mixture will be firm. When ready to use, set the bowl at room temperature until the mixture is not quite so firm.
Using a stand mixer with a whisk attachment, beat cold chocolate mixture until fluffy and spreadable.